This invention relates to an air purification device, and particularly but not exclusively to an air purification device which generates ozone.
By air purification device is meant a device which may be used to improve the quality of air, by removal of at least a portion of pollutants, odours, airborne particles, bacteria and/or the like from the air. Such a device may be used to clean exhaust gases from motor vehicles and other polluting processes, for sterilization purposes in xe2x80x9ccleanxe2x80x9d manufacturing facilities, hospitals, food storage and preparation areas, etc.
It is known that the introduction of ozone into air, typically by electrical generation of ozone, may be used as a way of purifying the air. Ozone is poisonous to bacteria and microorganisms, and also oxidises many organic compounds that may be present in odours, however as ozone is also toxic to humans, its use as an air purifier has had limited application.
Conventionally, ozone generators are operated using the xe2x80x9csilent dischargexe2x80x9d or xe2x80x9cdielectric barrier principlexe2x80x9d to convert oxygen into ozone. In such generators, the feedstock is typically oxygen gas because the use of air in a conventional device can lead to the production of undesirable end products such as oxides of nitrogen (e.g. NOx). The use of air as a feedstock can however be achieved using generators of the dielectric barrier type provided that they are operated at a high frequency. Such high frequency devices comprise two electrodes having a gap between them, air being passed through the gap so that electrical discharge across the electrodes causes the oxygen in the air to be converted to ozone. These devices generally have very narrow gaps between the electrodes and thus offer considerable resistance to the flow of air through the device, requiring a pump to provide airflow.
When such devices are used in air purification applications, they generally have to be left running for a long period of time (e.g. overnight) so that enough air can pass through the electrodes to be converted to ozone. The concentrations of ozone produced can be quite high and it is therefore necessary to prevent access to the area that is being purified. Conventional devices can thus only be used in confined spaces that can be controlled easily.
GB-A-2296172 discloses an ozone generator having dielectric spacers between the electrodes. The spacers maintain the spacing between the electrodes, which is important as the pressure between the electrodes can become raised above atmospheric pressure, causing the electrodes to bow or warp. The gap between the electrodes is of the order of a few millimeters or less, to help reduce the voltage required to establish a corona discharge. In some embodiments, an open dielectric foam, a dielectric filament material or irregularly spaced dielectric particles are placed in the electrode gap around the spacers, to provide an impedance to the flow of gas through the electrode gap so that gas flow between the electrodes and hence corona discharge is uniform. The air flow through the device is not high enough to enable the apparatus to be used as an air purification device.
The known devices are also often prone to overheating which reduces the efficiency of ozone production as ozone thermally degrades above about 50xc2x0 C. and can cause failure of the discharge electrode arrangement due to distortion such as bowing or warping.
It is known from U.S. Pat. No. 3,654,126 to use a fluidized bed of a dielectric such as sand to provide cooling to the electrodes and thereby increase the efficiency of ozone generation. The sand is fluidized by means of a high rate of gas flow passing between the electrodes. However the device described in this prior publication is bulky, expensive and mechanically complex. It is thus not suitable for use in air purification applications.
It is an object of the present invention to provide an air purification device which obviates or mitigates some or all of the problems mentioned with prior art devices.
According to a first aspect of the present invention there is provided an apparatus for purifying air comprising two electrodes having a dielectric material therebetween and means for applying a potential difference across the electrodes, wherein the electrodes are air permeable and the dielectric material is in the form of an air permeable, fixed bed and wherein means are provided to provide air flow through one electrode, across the fixed bed of dielectric material and through the other electrode.
According to a second aspect of the present invention there is provided a method of purifying air comprising passing the air through an air permeable electrode, an air permeable fixed bed of dielectric material and a second air permeable electrode whilst applying a potential difference across the bed to convert oxygen in the air to ozone.
The term xe2x80x9cfixed bedxe2x80x9d is intended to describe a material that extends between the electrodes so that it does not move in normal usage of the device but which is air permeable so that air may flow through gaps present therein. The term is intended to cover inter alia a bed of discrete particles, a foam, a sponge-like structure, and a bed of elongate elements such as filaments arranged in a contacting relationship with air gaps therebetween.
The invention causes air to flow through the air permeable electrodes and dielectric material, preferably in a direction parallel to the applied electric field, thus substantially increasing airflow through the device. This means that a simple fan may be used to push air through the device, rather than an expensive air pump. The air purification device of the present invention thus generates ozone with a high volume of airflow, but in low concentrations, eliminating problems of exceeding toxicity levels in xe2x80x9cpopulatedxe2x80x9d areas.
Cooling of the device, which increases the efficiency of ozone generation, is achieved by the use of a high air flow through the device, which is not possible with a solid, impermeable dielectric material, and without the need to fluidize a particle bed as described in the prior art. Cooling is thus achieved without expensive and bulky fluidization apparatus or air pumps or cooling water.
The use of the fixed bed also allows discharge to take at the points of contact of the dielectric (if the dielectric is in the form of discrete particles) so that the discharge across the electrodes is uniform. As a result, the spacing between the electrodes may be increased compared to the prior art devices, allowing greater airflow through the device. The fixed bed reduces level of airborne pollutants (especially particulates such as smoke, dust, soot, etc; aerosols; and bacteria) by electrostatic dust precipitation and by the chemical processes of ozonolysis, oxidation and sterilisation, and by the particles being burnt off due to electrical discharge at the points of contact of the dielectric.
The electrodes may be formed of a metal gauze or mesh. Suitable metals include steel and nickel.
The dielectric material may be any suitable material, but preferably has a dielectric constant less than 100, and more preferably less than 20. The dielectric material is preferably glass. The use of a material with a reasonably low dielectric constant, such as glass, allows for cost savings over dielectric materials having a higher dielectric constant, whilst still allowing the device to be efficient enough for use in air purification applications. Silica, alumina or another suitable dielectric (zirconia, sapphire, etc.) could be used in place of glass. Alternatively, materials such as barium titanate, which has a dielectric constant of 1000 may be used as the dielectric material, as it may be obtained at a relatively low cost.
Preferably the dielectric material is formed of a bed of discrete bodies in a contacting relationship. The discrete bodies are preferably particles, preferably regularly shaped particles and more preferably beads. The diameter of the beads is preferably about 1 mm to 6 mm. Glass wool, chips, or extruded foam could be used in place of beads provided that air permeability is retained and that elements of the dielectric material are in a contacting relationship, although regularly spaced beads give an advantage in that better airflow is allowed through the dielectric bed.
The potential difference applied across the electrodes may be Vpk-pk=10-20 kV and 10-15 kHz, although voltage such as mains at 50 Hz or 60 Hz could be used.